Solar cells may be defined as devices to convert light energy into electrical energy by using a photovoltaic effect of generating electrons when light is incident onto a P-N junction diode. The solar cell may be classified into a silicon solar cell, a compound semiconductor solar cell mainly including a group I-III-VI compound or a group III-V compound, a dye-sensitized solar cell, and an organic solar cell according to materials constituting the junction diode.
A solar cell made from CIGS (CuInGaSe), which is one of group I-III-VI Chal-copyrite-based compound semiconductors, represents superior light absorption, higher photoelectric conversion efficiency with a thin thickness, and superior electro-optic stability, so the CIGS solar cell is spotlighted as a substitute for a conventional silicon solar cell.
In general, a CIGS solar cell can be prepared by sequentially forming a back electrode layer, a light absorbing layer and a front electrode layer on a substrate including sodium. Since molybdenum (Mo) has a thermal expansion coefficient similar to that of an organic substrate with a low specific resistance, the molybdenum (Mo) is mainly used for the back electrode layer and the molybdenum (Mo) is deposited on the substrate through a DC sputtering process.
In the sputtering process for depositing the back electrode layer, if sputtering power is increased, tack time may be reduced, but the grain size is increased. Thus, compressive force may occur between grains, so the back electrode layer may be separated from the substrate. Meanwhile, if the sputtering power is reduced, the grain size is slightly reduced so the adhesive strength between the back electrode layer and the substrate may be enhanced, but the tack time may be increased.